The present invention relates to optical apparatus for measuring objects having a rectilinear profile.
For a better understanding of the state of the art and its inherent problems, conventional apparatus, illustrated in FIGS. 3 and 4 of the attached drawings, which has been produced to study the behaviour of a generic two-dimensional shape in the laboratory will first be described.
In FIG. 3 a He—Ne tube laser 1 delivers a beam 2 of collimated coherent light of uniform light density with a high degree of coherence. The beam 2 is expanded by a beam expander 3 having a pin-hole filter and directed onto an opaque object 4 the geometric dimensions of which it is desired to measure. By the diffraction principle described by Fresnel and Fraunhofer, at the points of interaction of the light wavefront with the opaque object 4, that is to say along its edge or outline, there are formed new wavefronts of spherical type the radial components of which are divergent with respect to the direction of the original incident beam (FIG. 4).
At this level, if the image were to be analysed in detail, there would be observed, in correspondence with the enlarged outline of the object 4, a region of uncertainty created by thin “fringes” of alternating light and shade which would render it difficult or impossible to form an exact determination of the spatial position of the edge.
The light beam 2, after having encountered the object 4, arrives at a spherical converging lens 5 disposed orthogonally of the direction of the beam.
According to the laws of geometric optics, only and exclusively the parallel components (indicated 2a) of the incident radiation (FIG. 4) converge at the focal point of the lens 5. At the focus of the lens 5 is disposed an obstacle 6, such as, for example, an opaque spot, hereinafter called a “stop” or “spatial filter” which impedes the propagation of the light.
The function of the spatial filter 6 is to stop only the parallel components 2a of the incident beam without interfering with the divergent and diffracted components 2b of the beam which can reach a focusing and enlarging converging lens 7 and finally be collected on a screen or photo sensitive chamber 8. The resulting image on the screen 8, after the spatial filtering just described, is constituted exclusively by thin lines of light which correspond to the outline of the object 4 standing out on a dark background.
The contrast between the illuminated line (useful signal) and the residual background illumination (noise) is greater the more the coherent light source satisfies the initial requirements of spatial homogeneity and parallelism.
The image thus processed lends itself particularly well to electronic analysis for measurement of the object. In fact, whilst it is impossible to establish exactly a criterion with which to choose a reliable and repeatable preferential measurement point on an undefined light/shade edge (such as would be that of the image obtained without the spatial filter), it is relatively simple to measure the distance, on the screen 8, between spaced lines, each of which has a very narrow maximum of luminous intensity.
Examples of theoretical studies which represent the known technique discussed above are described in the following publications: H. Lipson and C. A. Taylor Fourier Transform and X-Ray diffraction, 1958; G. Harnburn, C. A. Taylor, T. R. Welberry Atlas of Optical Transforms, 1975; F. Docchio, E. Sardini, O. Svelto, A. Taroni On-Line Dimensional Analysis of Surfaces Using Optical Filtering and Elaboration Techniques in the Fourier Plane, 1989; and R. G. Wilson Fourier Series and Optical Transform Techniques in Contemporary Optics, 1995.
However, the results achieved experimentally have been obtained in a laboratory using sophisticated optical and electronic instrumentation, in conditions very close to the theoretical ideal, that is to say:                it was possible to have available a light source having a high degree of coherence, typically that delivered by a He—Ne tube laser;        it was possible to filter as much as necessary, and expand the laser beam in such a way that the resultant beam had a uniform luminous distribution and the rays were parallel to one another; in other words it was possible to obtain a beam free from spatial harmonic components and having a flat wavefront;        it was possible to use high quality lenses and optics, with very large apertures with respect to the dimensions of the object under observation.        
Currently there are available on the market non-contact measurement devices (which utilise different principles from those described here) with which it is possible to obtain good measurement precision (typically 0.1 μm) but at high cost and with limited robustness of the instrument, or else, alternatively, to obtain economy but limited precision (typically not less than 5 μm).